Hydraulic alternators and compensators

The development of hydropower plants has been going on for over a century now. At the beginning of the last century, the unit power of the machines was relatively limited, but the increase in the output power of the alternators, and inevitably of the turbines, accelerated from the 1960s onwards.

One of the special features of hydraulic alternators is the uniqueness of each site: depending on the hydraulic conditions (flow rate and head of water) and the desired operating conditions, an optimum pair of parameters (power and speed) can be deduced for the site in question. As a result, the overall sizing of alternators can vary enormously: for example, a generator for a low-head, run-of-river head is quite different from one for a high-mountain dam. The associated turbines also feature very different technologies (Pelton turbines for high heads, Francis turbines for medium heads, and Kaplan turbines for low heads).

This great diversity is reflected in very different designs: a fast 600-rpm alternator is small in diameter and long, with the rotor fitted with a through shaft, while the slow 75-rpm alternator has a much flatter pancake shape, with a welded rotor center fitted with coupling plates at both ends.

The development of hydraulic alternators is also linked to the evolution of other energy sources and power grids. At the beginning of the last century, all the generators used in power plants were electricity-generating units. With the growth in power grid capacity and the development of nuclear power plants, new needs have emerged: the need to absorb part of the electricity produced during off-peak hours, due to the lack of operating flexibility of nuclear power plants, and the need to maintain and perfect grid stability.

Turbine-pump power plants were thus born. In the 1950s, these power plants were limited in terms of output, but with the development of nuclear power, their unit output increased considerably (depending on the country, for example in France from 1975 onwards, and later in China from the 1990s). The greater operating flexibility of nuclear power plants has subsequently slowed or even halted the development of pumped-storage power plants in Europe, but pumped-storage power plants are nonetheless one of the main sources of electricity storage.

Today, with the rising cost of fossil fuels, the need to maximize grid utilization and the problems associated with grid stability, the demand for higher performance (plant efficiency and productivity) justifies the development and implementation of variable-speed units. In industrialized countries, where hydroelectric potential can scarcely be increased, old units are being upgraded to more powerful units.

This...